Fluid-dynamic actuators are used quite commonly to move apparatus in many technical sectors like, for instance, on tire changing machines used to remove and mount tires from their rims.
These fluid-dynamic actuators comprise an external casing, normally cylindrical in shape, inside which a sliding chamber is obtained; inside this chamber a coaxial piston is mounted sliding which is operated by a fluid that is let into the sliding chamber and which makes the piston move in one direction or the other, pushing against its cross surface.
To the piston is associated at least one rod which extends outside the sliding chamber and which is designed to be hooked to an apparatus that must be operated by the actuator or that can be fixed to an unmoving point of a structure so that it is the casing that moves due to the reaction of the thrust of the fluid on the piston and that makes an apparatus hooked to it move.
The fluid normally used is pressurized air or oil. The force developed by the piston and which is transmitted to the rod, or, due to the reaction, to the casing, is determined, as is common knowledge, by the pressure exerted by the fluid and by the overall area of the piston on which said pressure acts.
Therefore, to be able to have strong forces available the surface of the pistons must be extensive and, therefore, the actuators must be proportionally large in size in relation to the forces one wishes to obtain.
This state of the art has a drawback which is the impossibility of obtaining high forces when, for example, for reasons of space the size must be compact, only small fluid-dynamic actuators can be used which as a result are only capable of providing forces of a limited extent with respect to the usages for which the fluid-dynamic actuators must be employed.
This drawback occurs, for example, in tire changing machines where all the actuating bodies of the various machine functions are located inside a box frame which forms its base.
In detail, this drawback concerns above all the actuator unit that operates the locking bodies used to block a wheel on the working platform that these tire changing machines are equipped with.
These locking bodies comprise a resting element whose resting surface can be substantially
horizontal or even vertical and against which a wheel is placed on which either maintenance or repair jobs are to be done.
A shaft extends centrally from this resting element, going through it axially and extending beyond it.
The centre hole of the wheel rim is inserted on one free end of this shaft, facing outwards.
This shaft is connected to a fluid-dynamic actuator unit located inside the base of the tire changing machine or it is the rod itself that extends from the piston of the latter and moves with a reciprocating motion either from or to the resting element.
The end of the shaft facing outwards is threaded and on this a conical body can be screwed which, after a wheel has been placed against the resting element having made the shaft pass through its centre hole, is screwed down by hand until it engages in this centre hole, adapting to it automatically thanks to its conical shape, until it rests against the edges of it.
In this phase the shaft is in a maximum upwards or outwards sliding position and, when the conical body is well positioned, the actuator unit is activated, exerting a traction force on the shaft to pull it along the direction of the resting element, thus locking the rim on it and, therefore, the wheel with the conical body.
The fact is though that the fluid-dynamic actuator unit, which can be housed in the base of the tire changing machine, must be small in size due to the small size of the base itself and of the numerous other devices the base houses. Therefore, this actuator unit is able to exert a limited force on the rims of the wheels even if these wheels are big and heavy like, for instance, those of transport vehicles, work site vehicles or agricultural vehicles.